I agree that it's part of the SV mythos, but honestly? It's a part of the mythos that's always troubled me, because I think it's really very simplistic. The idea that all any given person ever really needs is to be loved to keep them from doing or being bad is, IMO, a somewhat romanticized idea of the power of love that bears no relationship to the real world/real life.

No argument here. I think part of the problem is that TPTB just aren't good enough at really complicated storytelling to tell something that complex. I also think that perhaps they decided to stick with this particular motivation for Lex's fall because they thought it would be easier for a lot of the audience to accept, especially given Lex's popularity. (And I'm not sure that your desire to see Lex being responsible for the choices he makes towards his iconic destiny is widely shared in the fandom--given how frequently people still want to blame Clark's secret keeping, or Jonathan's judgmentalism, or Lionel's machinations for every bad decision Lex makes.)